Unprefix usage of -webkit-transform in tests

LayoutTests/css3/filters/effect-reference-subregion-hidpi-hw.html

 <script>
     if (window.testRunner) {
         testRunner.waitUntilDone();
         testRunner.setBackingScaleFactor(2, function() { testRunner.notifyDone(); });
     }
 </script>
 <svg xmlns="http://www.w3.org/2000/svg" width="0" height="0" version="1.1">
   <defs>
     <filter id="colormatrix" color-interpolation-filters="sRGB">
       <feColorMatrix x="10" y="20" width="80" height="60"/>
     </filter>
   </defs>
 </svg>
 <style>
 .filtered {
     -webkit-filter: url(#colormatrix);
     filter: url(#colormatrix);
     background-color: green;
     width: 100px;
     height: 100px;
-    -webkit-transform: translateZ(0);
+    transform: translateZ(0);
 }
 body {
     margin: 0px;
 }
 </style>
 <div class="filtered">
 </div>